Method of making extended surface heat exchanger



D. DALlN Oct. 4, 1955 METHOD OF MAKING EXTENDED SURFACE HEAT EXCHANGER 3Sheets-Sheet 1 Filed NOV. 13 1950 D. DALlN 3 Sheets-Sheet 2 Oct. 4, 1955METHOD OF MAKING EXTENDED SURFACE HEAT EXCHANGER Filed Nov. 13, 1950 I),Fill/ Oct. 4, 1955 DAMN 2,719,354

METHOD OF MAKING EXTENDED SURFACE HEAT EXCHANGER Filed Nov. 13, 1950 5Sheets-Sheet 3 i p......-.r--.-.../

United States Patent METHOD OF MAKING EXTENDED SURFACE HEAT EXCHANGERDavid Dalin, Stenkullen, Ronninge, Sweden, assignor t0 A/E SvenslraMaskinverken, Sodertalje, Sweden, a corporation of Sweden ApplicationNovember 13, 1950, Serial No. 195,201

3 Claims. (Cl. 29-1573) This invention relates to heat exchangers andrefers more particularly to extended surface for heat exchangers.

It has been conclusively shown that the most effective form of extendedsurface consists of a multiplicity of closely spaced relatively smalldiameter rod or wire-like elements joined to and extending from the basewall which separates the two media between which heat transfer is totake place. The advantage of this form of extended surface is discussedat length in Patent No. 2,469,635 issued to David Dalin and Gustav V.Hagby May 10, 1949.

As therein explained the wire or rod-like elements should be made ofmetal having high thermal conductivity, copper and aluminum beingspecifically mentioned. A commercially feasible method of joining theelement to the base wall is disclosed in the pending application ofDavid Dalin, Serial No. 82,572 filed March 21, 1949, now Patent No.2,584,189, granted February 5, 1952.

The one disadvantage of the extended surface disclosed in the aforesaidpatent and application is that copper exposed to air or gases containingoxygen oxidizes rapidly when its temperature exceeds 260 C., and Whileunder similar conditions aluminum has better resistance to oxidation theimprovement is not enough to cope with the elevated temperatureprevalent for instance in boiler furnaces and the gas passages thereof,and aluminum is not as satisfactory for the purpose as copper sincecopper has nearly twice the conductivity of aluminum and not as high accefiicient of expansion.

Also in situations involving large and rapid fluctuations in temperaturethe difference in coefiicients of expansion between steel or steelalloys of which the base wall is preferably made and the copper oraluminum of which the elements are made, placed so great a strain uponthe Welded junctions by which the elements are secured to the base wallthat these connections crystallized and failed.

Another limitation upon the use of copper for the extended surfaceelements is its inability to withstand the chemically reactive gasesprevalent in certain situations of use, but unless the extended surfaceelements are made of copper or aluminum, the only commercially feasiblemetals with the required heat conductivity, the advantages of this idealextended surface are lost.

The problem to which this invention is addressed is thus clearlydelineated, namely to find some way of protecting the extended surfaceelements against oxidation and corrosion without appreciably impairingtheir heat conductivity and to provide some means of preventingcrystallization and failure of the junctions by which the elements aresecured to the base wall. The obvious expedient of plating the elementswith appropriate metal to afford resistance to oxidation and corrosionat elevated temperatures is out of the question, if for no other reasonthan the cost involved since the plating would have to be done after allof the elements are welded to the base wall.

However, notwithstanding the high cost involved, plating with variousmetals having better oxidation and cor- Patented Oct. 4, 1955 rosionresistance than copper (in air) was tried and found unsatisfactorybecause of the ditficulty of obtaining a good protective plating at theroots of the elements where they are welded to the base wall and becausethe plating cracked when the elements were bent into position. Alloyingthe copper to improve its resistance to oxidation and corrosion is alsoout of the question for alloying greatly reduces the heat conductivityof the metal.

It was evident, therefore, that unless a solution to these problems wasfound, this ideal extended surface would be relegated to the lessrigorous service of such ancilliary apparatus as economizers and airpreheaters where the temperatures involved are not too high, and toservice where the gases are not too chemically reactive.

This invention meets and solves these problems.

In broad terms the invention achieves its purpose by making the elementsbimetallic with a core of copper or aluminum and a sheath of metalhaving a resistance to oxidation in air and gases containing oxygen atelevated temperatures, and a resistance to corrosion by chemicallyreactive gases, better than that of copper. Steel of the same type usedfor the base wall or tubes to which the extended surface elements aresecured is entirely satisfactory for most purposes, but the unequalcoefficients of expansion of steel and copper or aluminum introducedanother problem which was discovered when the bimetallic elements werefirst applied to the base wall. It was found that the high heat involvedin the resistance welding operation by which the elements are attachedto the base wall caused the sheath to crack and burst at the root of theelement.

It was also found that when the extended surface elements were subjectedto sudden and large temperature rises particularly when often repeatedand more so when the heat was supplied via the base wall to the extendedsurface elements, the sheath would crack and burst along shorter orgreater portions of its length.

Upon analysis it was determined that the reason for such cracking andbursting of the sheath was the fact that the core metal expanded so muchmore rapidly than the steel sheath that it cracked and broke the sheath.This problem is overcome in this invention by annealing the bimetallicelements before welding them to the base wall. By annealing the elementsat a temperature at least as high as that encountered in normal serviceall evidence of cracking and bursting of the sheath was eliminated.

Thus, as will appear more fully hereinafter the present inventioncompletely solves the problems to which it is directed in a practicaland simple manner which will be clear from the following description,reference being had to the accompanying drawings which form a part ofthis specification, and in which:

Figure 1 is a cross sectional view through a heat exchanger constructedin accordance with this invention and especially adapted for use insteam boilers since the extended surface is mounted on the outside of atube through which boiler fluid may flow;

Figure 2 is an enlarged detail view showing a portion of the tubularbase wall of Figure 1 and one of the extended surface elements prior toits attachment to the base wall;

Figure 3 is an enlarged detail view similar to Figure 2 but showing theextended surface element attached to the tubular base wall;

Figure 4 is a detail sectional view at still a larger scale toillustrate particularly the junctions between the core and sheath of theextended surface elements and the junction of the extended surfaceelements to the base wall;

Figure 5 is a cross sectional view through a portion of another form ofheat exchanger constructed in accordance with this invention and adaptedparticularly for installations wherein the fluid medium at one side ofthe base wall has substantially the same surface conductance as thefluid medium at the other side thereof so that extended surface shouldbe used on both sides of the base wall;

Figure 6 is a cross sectional view through still another form of heatexchanger constructed in accordance with this invention anddistinguished from those of Figures 1 and 5 in that the extended surfaceelements are applied tangentially to a tubular base wall;

Figure 7 is a plan view, on a reduced scale, of the heat exchanger shownin Figure 6;

Figure 8 is a cross sectional view through a portion of the heatexchanger shown in Figure 1 but illustrating a different constructionfor the extended. surface elements;

Figure 9 is a cross sectional view through a heat exchanger of the typeshown in Figure l but in which all of the exposed surfaces areadditionally protected against oxidation and corrosion by a coating ofvitreous or other suitable oxidation and corrosion resisting material;

Figure 10 is a. detail sectional view longitudinally through the outerend portion of one of the extended surface elements to show the end ofits core covered by a cap' of the same metal as its sheath;

Figure 11 is a cross sectional view through a portion of. a so-calledwater wall of a furnace illustrating another adaptation of thisinvention;

Figure 12 is a plan view of a portion of such wall;

Figure 13 is a cross sectional view through Figure 12 on the plane ofthe line 1313 but on an enlarged scale; and

Figure 14 is a detail cross sectional view through Figure 12 on theplane of the line 14-14.

Referring now more particularly to the accompanying drawings in whichlike numerals indicate like parts, the numeral 5 designates in all formsof the invention shown the metal base wall which separates the two mediabetween which heat' transfer is to take place and to which the extendedsurface elements designated generally by' the numeral 6 are attached.Depending upon the service for which the heat exchanger is designed thebase wall is either tubular as in Figure l or planar as in Figure 5, butin all instances the base wall is of metal having high tensile strengthand a relatively low coefficient of expansion.

For most installations mild steel is ideally suited to thepurpose butwhere extremely oxidizing or corrosive conditions prevail a suitablesteel alloy, as for instance a chromium steel alloy relatively high inchromium but low in carbon content, should be used and, if desired, thebase wall may be laminated with only that surface thereof exposed to theoxidizing or corrosive conditions formed. of the more expensive alloyand the other surface formed either of'mild steel or even a cheapersteel or other metal.

For purposes of illustration such a laminated base wall is illustratedin Figure 6 wherein the tube 5 has a core 7 of relatively inexpensivesteel or other suitable metal within a shell 8 of metal such as chromiumsteel alloy which has high resistance particularly to oxidation.

The extended surface elements indicated generally by the numeral 6 areattached to the base wall either endwise, as shown for instance inFigure l, or tangentially as shown in Figure 6. In each instance theextended surface elements are bimetallic and in all embodiments of theinvention shown with the exception of that of Figure 8 the extendedsurface elements consist of a core portion 9 and a sheath portion 10;The core is of metal having high thermal conductivity and arelatively'high coefficient of expansion. Copper is ideally suited forthe purpose although aluminum may be used.

The sheath 10-is of metal having high tensile strength and a relativelylow coefficient of expansion but most important possessing goodresistance to oxidation and corrosion. at. elevated temperatures. Themetal of which the sheath 10 is formed must possess better resistance tooxidation in air at elevated temperatures than copper and the metalshould be highly resistant to corrosion regardless of temperature. Thismeans that the sheath metal should be proof against oxidation andscaling to temperatures of at least 400 C. and in some cases as high as800 C.

Another requirement of the metal used for the sheath is that it havesubstantially the same coefficient of expansion as the metal of the basewall or at least that surface of the base wall to which the element isattached. Thus, if the base wall is of mild steel the sheath metallikewise preferably should be of mild steel and if the base wall is madeof a steel alloy the sheath likewise preferably should be made of asteel alloy.

Another metal that can be advantageously used for the. sheath isAdmiralty bronze which is highly resistant to oxidation andcorrosion andhas an expansion coelficient close to that of steel.

The wall thickness of the sheath may vary but generally should be nogreater than required by the manufacturing process used to produce thewire from which the element is made, and/or no greater than necessary tosecure a good. and durable bond between the sheath and the base wall andgenerally assure durability of the sheath under. prevailing serviceconditions. For elements of average diameter which may range between 2mm. and 8 mm. the wall. thickness of the sheath should be approximatelybetween .2 mm. and .7 mm.

The specific manner in which the elements are produced, that is themanner in which the core is enshroudedwith the sheath, forms no part ofthis invention and may be done in any of the ways known to the art ofproducing bimetallic wire. Thus, for instance, an ingot may be preparedhaving a core of copper or aluminum, depending upon which of thesemetals is to be used for the core, and then progressively reduced indiameter and elongated as by rolling and wire-drawing until. a wire ofthe desired diameter is obtained. Such diameter reduction andelongation, of course, forces the sheath into firm intimate grippingrelation with the core.

The specific way in which the ingot is prepared also is of no directconcern to this invention. Thus, the ingot may be produced by castingthe core metal within a tube of the sheath metal, or the sheath metalmay be wrapped about a bar of the core metal with its longitudinal seamsecured by welding.

Regardless of how the wire from which the extended surface elements areformed is manufactured it has been found to be extremely important thatbefore the wire is attached to. the base wall either as rod-likeelements secured endwise to the wall or as wire-like members securedtangentially to the base wall, the wire must be annealed. Experience hasdemonstrated that unless it is annealed the sheath cracks and burstsduring the welding operation, where the heat of the welding operation isconcentrated. This no doubt follows from the fact that because of thedifference in expansion coeflicients between the core metal and sheathmetal the rapid temperature rise so stresses the sheath metal as tocause its failure- Thev gradual increase and decrease in temperature ofthe annealing operation, however, does not set up these strains but onthe contrary conditions the wire to preclude rupture or failure of thesheath during subsequent temperature changes in use.

While the exact explanation of how this conditioning is achieved byannealing. the wire is not known it is quite probably that during theannealing operation the greater expansion of the core expands the sheathbeyond its elastic limit, and since the temperature employed in theannealing should be at least that encountered in the average use forwhich the heat exchanger is designed, the. diameter to which the sheathis expanded during the annealing operation is not exceeded in service.During the cooling step of the annealing operation the core, of course,contracts more than the sheath and thus tends to pull away from thesheath. This creates a zone in which the molecules of the metal of thecore and sheath pull in opposite directions, and this zone apparentlyhas suificient elasticity to accommodate any subsequent expansion andcontraction of the core without imposing bursting stresses upon thesheath.

It is also possible that a minute clearance between the core and thesheath may be the result of the annealing operation. However, there hasbeen no noticeable reduction in the heat transfer from the sheath to thecore.

Hence, this zone which lies at the contiguous surfaces of the core andsheath can be considered a cushioning zone to cushion the stressesinvolved in subsequent expansion and contraction of the element.

The explanation of why annealing conditions the wire to precludebursting of its sheath in service may also lie simply in the fact thatannealing renders both the core and sheath ductile, but whatever theexplanation may be, as pointed out before, it is important that the wirebe annealed before it is attached to the base wall.

In cases where the extended surface elements during use are subjected toextremely high temperature or where the lack of affinity between themetals in the core and the sheath tend to lessen the bond between thecore and the sheath a thin layer of a third metal or other materialhaving suitable bonding, flexibility or firmability properties may beintroduced between the core and the sheath in order to thereby increasethe effectiveness of the cushioning zone.

If the wire is attached endwise as in Figure 1 it is, of course, cutinto rod-like elements of the appropriate length and these elements arethen resistance welded in the manner described in the aforesaid pendingapplication, Serial No. 82,572. In conformance with the method theredescribed the elements and base wall are connected in an electriccircuit and the elements are pressed against th base wall whileconnected in a resistance welding circuit; and if the exchanger is ofthe type shown in Figure 1 the elements are applied radially to the tubeas indicated by construction lines in Figure 1 and then subsequentlybent into parallelism.

As explained at length in the aforesaid copending application, thewelding time, welding current, conductivity of the element, conductivityof the base wall and the welding force or pressure are critical factors.However, since the sheath of the elements is of the same metal as thebase wall or at least has substantially the same coefiicient ofexpansion as the base wall, the securement of the element to the basewall is considerably easier than when the junction depends solely uponthe welding of dissimilar metals as for instance copper to steel. As aresult crystallization and failure of the junctions which has beenexperienced with units in which the elements were entirely of copperwelded to a steel base wall is entirely overcome by this invention,since in welding the elements to the base wall the sheath metal fusesperfectly with the metal of the base wall and no disruptive forces existin service to cause crystallization and failure.

Whether or not the core of the element also fuses with the base wall orbecomes securely welded thereto is, therefore, no longer of importancesince the junction of the sheath to the wall is fully capable of holdingthe element in place and it should be observed that whether the core isfused to the base wall or not it is in such intimate contact therewithas to assure good heat conduction therebetween.

A comparison of Figures 2 and 4 illustrates the before and afterconditions of the element and base wall and attention is directedparticularly to Figure 4 which shows how the metal of the sheath isfused to the metal of the base wall. This view also indicates more orless 6 diagrammatically the cushioning zone between the sheath and coreby which expansion and contraction of the core without imposing burstingstresses upon the sheath is made possible.

In that form of the heat exchanger illustrated in Figures 6 and 7wherein the extended surface is better defined as a series of wiressecured tangentially to the tubular base wall, the present inventionmakes this form of heat exchanger commercially possible since itobviates the difficulties that were experienced in the past due to theclose proximity of the welded connections. Whereas in the past it wasnecessary to individually weld each junction it is now possible tosimultaneously weld a great number of the junctions so that an entiremat of wire elements can be secured to a plurality of tubes in onewelding operation.

As has been seen, one of the problems solved by this invention is thatwhich caused the crystallization and failure of the junctions by whichthe elements are secured to the base wall. As shown in Figure 8, thisadvantage can be attained even though no steel sheath is employed.Situations where it is desirable to have the exposed surfaces of theelements of copper thus can be accommodated. In this case the elementsare provided with a small diameter core of steel or other metal high intensile strength and having substantially the same coefficient ofexpansion of the base wall, within a body of copper. When such anelement is welded in position the steel core fuses with the metal of thebase wall to achieve the desired strength at the junction even thoughthe body of the element is very nearly all copper.

Though the art has generally understood the advantages of coating metalsurfaces with a layer of vitreous or other material possessing goodresistance to oxidation and corrosion to protect the same againstoxidation at high temperatures, before the advent of the extendedsurface heat exchanger of the aforesaid Dalin et al. patent with itsgreat reduction in tube requirements and hence far fewer connections tobe made between tubes and headers, the use of such protective coatingswas not practically feasible.

The obviously greater difficulty of connecting a vitreous enamel orglass covered tube to a header and the attendant cost ruled out thispractice, but while the improved extended surface of the aforesaidpatent eliminated the reason for not using a protective coating ofvitreous or other protective material, it substituted an even greaterbarrier to its adoption. Such protective materials can be produced witha coefiicient of expansion which matches that of the metal to which itis to be applied but it cannot be rendered compatible in this respectwith more than one metal.

Therefore, since the extended surface of the new heat exchanger wascopper and the base wall steel, a vitreous or similar coating wasimpossible. This invention, however, for the first time makes itfeasible and possible to coat the entire surface of the heat exchangerwith a vitreous or other suitable protective covering since all theexposed surfaces which are to be covered have the same coeflicient ofexpansion. Thus where the added protection of such a coating is desired,it may be applied, and Figure 9 shows such a coating 12 extendingcontinuously over the entire exposed surface, including the ends of theextended surface elements.

Heating surface so protected has the tremendous advantage of beingpractically universal in its utility. With it there is no need forspecial temperature controls to pre- Vent condensation of chemicallyreactive gases upon the heating surfaces which means, of course, thatthe dew point problem disappears. Also such a coating adapts thestructure to use in acid and strong alkaline liquids.

Where the surface is not coated with vitreous or other protectivematerial, the ends of the elements, if desired, may'be pinched to closethe sheath material over the end of the core, or a separate cap 13 maybe applied as 7 shown in Figure 10, but since the cross sectional areaof the core'is very small this precaution is ordinarily not required.

Another application of this invention, illustrated in Figures 11 to 14,inclusive, is for a so-called water wall for'the furnaces of steamboilers and the like. As brought out, for instance, in the patent to T.E. Murray No. 2,220,579 issued November 5, 1940, it has been customaryin the prior art to line the walls of furnaces with water tubes, and toreduce the number of tubes required laterally projecting fins have beenwelded thereto. This invention is, therefore, admirably suited to thispurpose since it enables far greater spacing of the water tubes than hasbeen heretofore possible.

To this end the tubes through which the boiler fluid circulates havefins 6' welded thereto in the same fashion as the extended surfaceelements are attached to the base Wall in the embodiments of theinvention described. The fins 6 comprise a core 9' of copper or othermetal of high thermal conductivity and a sheath 10 of steel or othersuitable metal having the same coefficient of expansion as the tubes. Itis, of course, important here as in the case of the wire-like elementsthat the fins be annealed before their attachment to the tubes.

The advantage of this construction over the steel fins is illustrated bycomparison of the length of the bimetallic fins with the approximatelength of steel fins (indicated by the broken lines L in Figure 13) asused for this purpose in the prior art. Obviously with this constructionthe same protection for the furnace walls is obtained with far lesstubes than was heretofore necessary.

It is, of course, to be expected that enshrouding the elements in themanner described will detract somewhat from their ability to transferheat to and from a medium flowing thereover, but this slight reductionin surface conductance is negligible compared to the tremendousadvantage which this invention achieves. By actual test it was foundthat as between elements formed entirely of copper and those constructedin accordance with this invention, there is a reduction in heat transferof only six percent.

From the foregoing description taken in connection with the accompanyingdrawings, it will be readily apparent to those skilled'in the art thatthis invention is of tremendous importance to the heat exchange artgenerally and especially to that branch of the art dealing with steamboilers and ancilliary apparatus since it provides the means by whichall of the advantages of the aforesaid Dalin et al. patent and thecopending application may be achieved in a commercially economical'andentirely practical manner.

What 1 claim as my invention is:

1. In the method of permanently attaching extended surface elongatedelements of metal having heat conductivity and coetficient of expansionvalues on the order of those of copper, to heat exchanger tubes having abase wall of ferrous metal of considerably lower heat conductivity andcoefiicient of expansion values than those of the extended surfaceelements, without the necessity of fusing the elements to the base wall,the steps of: tightly encasing each extended surface element in a thinattaching sheath of ferrous metal which is corrosion resistant at hightemperatures and has substantially the same coefficient of expansion asthat of the base wall; annealing the thus encased extended surfaceelements; and by resistance welding securing the attaching sheaths ofthe annealed elements to the base wall to provide an uninterrupteddirect all metal heat conducting path between the base wall and theareas of the elements closest thereto and with the encased elementsprojecting outwardly from the base wall, so that the desired good heatconducting and 7 mechanically secure connection between the elements andthe tubes is effected without the need for actual fusion ofthe elementsthemselves to the base wall as a consequence of the welded jointsbetween their attaching sheaths .and the base wall, regardless ofwhether theelments are applied radially endwise to the heat exchangertubes or tangentially of said tubes.

2. In'the method of permanently attaching extended surface elongatedelements of metal having heat conductivity and coefficient of expansionvalues on the order of those of'copper, to heat exchanger tubes having abase wall of ferrous metal of considerably lower heat conductivity andcoefiicient of expansion values than those of the extended surfaceelements, without the necessity of fusing the elements to the base wall,the steps of: tightly encasing the entire length of each extendedsurface element in a thin tubular attaching sheath of ferrous metalwhich is corrosion resistant at high temperatures andhas substantiallythe same coetficiento'f expansion as that of the base wall; annealingthe thus encased extended surface elements; and'by resistance weldingsecuring one end of each attaching sheath to the base wall with theadjacent exposed end of the annealed element directly abutting the basewall in intimate heat conducting relationship therewith, so' that thedesired good heat conducting and me chanically secure connection betweenthe elements and the tubes is effected without the need for actualfusion of the elements themselves to the base'wall as a consequence ofthe welded joints between their attaching sheaths and the base wall. i

3. In the method of permanently attaching extended surface elongatedelements of metal having heat conductivity and c'oefiicient of expansionvalues on the order of those of copper, to a plurality of spaced apartheat exchanger tubes having a base wall of ferrous metal of considerablylower heat conductivity and coefiicient of ex pansion values than thoseof the extended surface elements, without the necessity of fusing theelements to the base wall, the steps of: tightly encasing the entirelength of each extended surface element in a thin attaching sheath offerrous metal which is corrosion resistant at high temperatures and hassubstantially the same coefiicient of expansion as that of the basewall; annealing the thus encased extended surface elements; and byresistance welding securing the attaching sheaths of the annealedelements to the base wall, with the encased elements substantiallyparallel to one another and extending transversely across the tubestangentially thereof, to provide an uninterrupted direct all'metal heatconducting path between the base wall and the areas of the elementsclosest thereto so that the desired good heat conducting andmechanically secure connection between the elements and the tubes iseffected without the need for actual fusion of the elements themselvesto the base wall as a consequence of the welded'joints between theirattaching sheaths and the base wall.

References Cited in the file of this patent UNITED STATES PATENTS 47,940Farmer et a1. May 30, 1865 1,140,136 Eldred May 18, 1915 1,512,961 WeilOct. 28, 1924 1,603,491 Osnos Oct. 19, 1926 1,607,968 Spire Nov. 23,1926 1,653,378 Steel Dec. 20, 1927 1,802,695 Bennett Apr. 28, 19311,821,702 Freeman Sept. 1, 1931 1,884,741 Kleffel; Oct. 25, 19321,929,444 Murray Oct. 10, 1933 2,064,141 Askin Dec. 15, 1936 2,088,446Specht July 27, 1937 2,149,696 Holmes Mar. 7, 1939 2,154,448 Hotfer Apr.18, 1939 2,308,319 Stanton Jan. 12, 1943 2,337,294 Cooper Dec. 21, 19432,433,687 Durst a. Dec. 30, 1947 2,450,203 Morgan Sept. 28, 19482,584,189 Dalin Feb. 5, 1952

1. IN THE METHOD OF PERMANENTLY ATTACHING EXTENDED SURFACE ELONGATEDELEMENTS OF METAL HAVING HEAT CONDUCTIVITY AND COEFFICIENT OF EXPANSIONVALUES ON THE ORDER OF THOSE OF COPPER, TO HEAT EXCHANGER TUBES HAVING ABASE WALL OF FERROUS METAL OF CONSIDERABLY LOWER HEAT CONDUCTIVITY ANDCOEFFICIENT OF EXPANSION VALUES THAN THOSE OF THE EXTENDED SURFACEELEMENTS, WITHOUT THE NECESSITY OF FUSING THE ELEMENTS TO THE BASE WALL,THE STEPS OF: TIGHTLY ENCASING EACH EXTENDED SURFACE ELEMENT IN A THINATTACHING SHEATH OF FERROUS METAL WHICH IS CORROSION RESISTANT AT HIGHTEMPERATURES AND HAS SUBSTANTIALLY THE SAME COEFFICIENT OF EXPANSION ASTHAT OF THE BASE WALL; ANNEALING THE THUS ENCASED EXTENDED SURFACEELEMENTS; AND BY RESISTANCE WELDING SECURING THE ATTACHING SHEATHS OFTHE ANNEALED ELEMENTS TO THE BASE WALL TO PROVIDE AN UNINTERRUPED DIRECTALL METAL HEAT CONDUCTING PATH BETWEEN THE BASE WALL AND THE AREAS OFTHE ELEMENTS CLOSEST THERETO AND WITH THE ENCASED ELEMENTS PROJECTINGOUTWARDLY FROM THE BASE WALL, SO THAT THE DESIRED GOOD HEAT CONDUCTINGAND MECHANICALLY SECURED CONNECTION BETWEEN THE ELEMENTS AND THE TUBESIS EFFECTED WITHOUT THE NEED TO ACTUAL FUSION OF THE ELEMENTS THEMSELVESTO THE BASE WALL AS A CONSEQUENCE OF THE WELDED JOINTS BETWEEN THEIRATTACHING SHEATHS AND THE BASE WALL, REGARDLESS OF WHEATHER THE ELEMENTSARE APPLIED RADIALLY ENDWISE TO THE HEAT EXCHANGER TUBES OR TANGENTIALLYOF SAID TUBES.